Solar or photovoltaic cells are manufactured by depositing conductive inks in desired patterns on the tops and bottoms of a solar cell wafer. The wafers are thermally processed in a furnace system to dry the conductive ink and burn off binders and other materials and then to fire the materials to form metallization patterns on the wafer surfaces. Furnace systems for such metallization processes typically employ infrared heating to provide the rapid thermal processing environment needed for processing the wafers.
Known wafer firing furnaces can be generally characterized as comprising three sections: a drying zone at an entrance where wafers are loaded into the furnace, a burnout/firing zone, generally thought of as the middle zone, and a cooling section located at the end and having an exit from which the wafers are removed. In many wafer firing furnaces, the conveyors used for processing the wafers through the sections are usually single-belt structures where the same belt is subjected to the different temperatures according to which section the portion of the belt is located.
A furnace with only a single conveyor belt running through its entire length, however, provides a process that has only one rate of speed for passing the wafers through the system. Accordingly, this single rate of speed limits the thermal profile experienced by a wafer as it passes through the furnace on the conveyor belt. If it is necessary to change the amount of time a wafer spends within a section of the furnace, the belt speed must be increased or decreased, which then also changes the duration the wafer experiences in the other sections of the system.
It is known that a second conveyor belt and drive may be placed in the burnout/firing zone of the furnace, in order to shorten the duration of the temperature spike without reducing the temperature peak, by running the second conveyor belt at an increased speed as compared to the speed in the other zones of the furnace. Providing a second belt, however, presents issues as to how to either receive or transfer a wafer from/to a belt that is running at a different speed, i.e., relatively slower or faster, than the current belt, without damaging the wafer in the process.
What is needed is a wafer processing furnace that provides flexibility for altering the thermal profile in various sections of the furnace so that each furnace segment may be individually adjusted without adversely affecting another furnace section.